There is nothing more to it. It is the law. I cannot stress this point enough. Destroying the electronic devices was not optional.
It was not a PR move at all. In testimony the Guardian even admitted it was not a PR move. It was necessary.
[Source [Video] http://www.theguardian.com/uk-news/2014/jan/31/footage-relea...]
EDIT TO ADD: I really don't care if you think I am a shill shrug - that says more about you and your tolerance of new facts than it says anything about me. The source I posted is from The Guardian, about the Guardian with video testimony from the Guardian. It couldn't be further from GCHQ to be honest.